Taking a step further back, there are a
number of campaign-level decisions that
a GM can make and enforce to provide a more
lethal environment at the campaign / setting level by design
rather than by implementing specific combat resolution options.

RATIO OF DEF TO DAMAGE CLASSES

The HERO System favors a defender over an attacker, and this
bias is perhaps most significantly expressed by the relative
expected values of defenses vs damage.

The math behind this is simple; every 1
increment of resistant damage subtraction (DEF) cancels
the average BODY damage result of 1 Damage Class (DC),
with DC's gaining a slight edge every
6 DC.

However 1 DEF costs 3 points while
1 DC costs 5 points.

Thus while 6 DC costs
30 points for an attacker, a defender could
have 10 DEF for the same 30 points.

An attacker also has to hit in the first place,
with a chance of failure.

And to further complicate things, in HERO System
combat defenders do not have to statically stand
around on the attackers turn waiting to get hit.
There are a plethora of defensive
options available to avoid or reduce damage
at the cost of an action such as Abort to Dodge
or Dive For Cover.

The net result of this is that if two characters
have equal points spent on attack and defense,
the attacker needs to successfully hit the defender
and also roll high above average to do direct harm
(though secondary / collateral harm might
still be inflicted such as Stun, Knockback or Knockdown,
or an effect such as a trip).

All of this obviously strongly favors the defender.

I can't stress this enough; in my opinion
the central dynamic of relative average DEF to average DC
is the single most important consideration for tuning lethality
for a given campaign and thus should be carefully monitored
by the GM for all HERO System campaigns.

LOWER DEFENSES

The simple expedient of keeping a very tight
reign on defenses (particularly
resistant defenses) available within a campaign
results in an immediate
increase in lethality, independently of
any other factor. In many ways this option
is a force multiplier for lethality as it
will make any other options employed even
more lethal.

By enforcing one or more campaign restriction
affecting defenses, the GM can easily shift the intrinsic bias of the HERO System
from favoring the defender towards a more neutral
bias, or even further over to favor the attacker.

Some of the ways a GM can accomplish this are listed below:

Flat Total DEF Cap: a simple approach is to just set a flat limit on total DEF allowed to characters. I don't recommend this approach as it fails to take into account nuances, but it has the virtue of simplicity.

Real Cost Cap: the GM can place a cap on how many character points can be spent on defensive abilities. Like all point caps, this comes at the cost of extra bookkeeping, so it is not without overhead. This also has limited impact in a gear does not cost points setting.

Active Points Cap: the GM can place a cap on how many active points defensive powers can have. Beyond the normal bookkeeping hassles, this penalizes the Hardened Advantage which can be exploited.

Combat Luck Restriction: either disallow Combat Luck (and similar non-gear DEF granting abilities) or impose a cap on it. I typically do this in a gear does not cost points setting.

Gear List: a very direct and effective approach in a gear does not cost points setting is to amend the list of available Armors to not include higher DEF items.

Market Forces: a similar but more nuanced approach to modifying the gear list is to arrange things so that higher DEF Armor is either less available or is prohibitively expensive to buy / maintain, or both. This is generally my preferred approach as it is very "natural", but it puts the onus on me as the GM to be consistent across the duration of a campaign.

A side note on campaign caps in general, is that I prefer to avoid them due to the extra bookkeeping
and constant math checking on characters to ensure that they conform to the limit. There is also
a strong tendency for campaign maximums to also become defacto minimums as players will have a natural
tendency to prioritizing buying up their abilities to the limit "to be competitive". If no cap is formally
declared and instead more subtle means are used to disincentivize stat inflation, this
sort of "keeping up with the joneses" creepage is avoided.

EXAMPLE OF REDUCED ARMOR AVAILABILITY

Consider the real world for a moment.
Guns are much more common and available than
body armor of any sort, with the most common
form of body armor being a so-called "bullet proof
vest" which provides only very limited protection
at best, and which is usually not commonly available to the
average person.

Since almost no one actually has any body armor,
the bias is strongly in the favor of an
attacker weilding a lethal weapon since
most hits are going to inflict full damage
with no mitigation. As modeled in the HERO System, in a
setting simulating a real world distribution of weapons and
body armor, even a 1d6k weapon such as a small caliber pistol
or knife is dangerous, and the typical 2d6k "gun"
is absolutely deadly.

Within a fantasy context, in most "higher"
types of fantasy armor is pretty common,
with most people having the equivalent of
around DEF 6 armor and heavier armored characters
creeping up around 9 to 15+ depending on
the prevalence of magic items, abilities
like Combat Luck, and special "feats"
/ heroic knacks / "super skills".

In such an environment 2d6k attacks are
weak; it takes at least 4d6k to be reliably
lethal and only a 5d6k or higher attack
is really scary to the average character.
In "lower" types of fantasy where
armor more serious than DEF 1 or 2 "leathers"
is scarce and even DEF 5 or 6 "chainmail"
or equivalent is rare a typical 2d6k attack
is viable again, and a 3d6k attack is reliably
lethal and frightening.

INCREASE DAMAGE

The obvious corrolary to decreasing defenses
is to increase the damage of attacks. The
basic math of DC vs DEF applies here, and it is perhaps
the more intuitive approach to pursue when seeking to
increase lethality. Also, human nature tends towards the fallacy that
more is always better and thus there is a tendency to prefer options
that add rather than options that take away.

However, this can be a rather naive and problematic approach as there
are other side effects caused by raising average damage that
decreasing defenses by an equivalent amount does not incur.
I briefly touch on many of these side effects below.

CONS OF INCREASING DAMAGE

Invalidates Equipment Lists:
the existing equipment lists are effectively
useless and either need to be rewritten
from scratch or tossed out in favor of custom
weapons that do heightened damage.

Material World Issues:
objects and materials in the campaign world effectively
become much more "brittle"
since attacks are doing more damage, which
raise weird scenarios where a character
uses a dagger to carve through a castle
wall and so forth; the "Real Weapon"
Limitation taken on most weapons gives GM's
an "out" to not permit this sort
of silliness, but it will force the GM to
make many more judgement calls along these lines
across the span of a typical campaign and
can lead to arguments and resentment from
gamist players who feel that if the mechanics
allow them to do something they should be
able to do it even if it doesn't make
rational "sense" based upon simulated
"reality".

Batter Up!:
Knockdown and Knockback become much more
prevalent and significant.

Yahtzee!:
More dice thrown means more dice counted
which means more time spent on combat.

Variance Is A Bitch:
As the number of dice grows, statistical
variance increases resulting in wider "swings"
in combat; i.e. more random and less predictable
resolution. An entire combat can be ended
with one extreme damage roll.

Overly Advantaged Advantages:
The impact of Power Modifiers such as AP and Penetrating
on attack Powers that increases the effect of each die's results
increases as more dice are being rolled.

Bigger Booms:
AoE's and Explosions get bigger and
thus more attractive and also more time
consuming to resolve.

Damage Is King:
The importance of tactics other than "strike first, strike last, strike hardest"
and many non-damaging abilities is sharply reduced.

Flat Cost Beats Scaling Cost Mitigation:
The utility of a few flat-cost abilities
priced against the normal expected damage
spread are greatly increased in effectiveness vs scaled
DEF. Abilities that allow complete evasion
of damage such as Invisibility and Desolid
are the most obviously improved options. Damage Reduction
becomes very efficient for its cost. Missile Reflection
improves considerably in utility. DCV is strongly increased
in importance over PD, ED, PowD, and FD.
And so. This dynamic is very significant.

I'm A Mobile Weapon Platform:
Characters can become little more
than damage delivery mechanisms. "Alpha
strikes", whereby characters unleash
their biggest, most dangerous attacks at
the onset of an altercation with the goal
of wiping out their opponents before they
themselves can act, become a common occurance.

I could go on. The point is, while the core
DC vs DEF math is the same when considering
decreasing defenses or increasing damage,
the later option is something I generally don't recommend
as it tends to make the game LESS realistic
rather than MORE realistic, which is usually
part of the goal of upping deadliness.

Having said that, in the context of the Fantasy genre,
there is a scenario where upping damage can be beneficial for
modeling a particular feel. Namely, some very "High" Fantasy
settings have more in common with the Supers genre than the gritty
settings of Conan or the Grey Mouser.

So-called "Plate And Capes" Fantasy often features larger than life
characters with incredible superhuman abilities. In that sort of setting
running the game using the rules for Supers, with gear bought as Powers
and increased Active Point limits will result in characters throwing around
big damage effects and mechanically will play much the same as a Supers
campaign with just a cosmetic difference.

INCREASED KILLING DAMAGE MULTIPLE

A variation on increasing damage that avoids
many of the kinds of side effects I mention
above is, if using Hit Locations, to increase
the multiples for body damage to some or
all locations. For instance, Head Shots
are normally x2 BODY; this can be increased
to x3, x4, x5 etc. (whatever fills your
sails and feels "right" for the
level of lethality you are looking for).

The side effect of this method is that sectional
defense for areas where the multiple has
been increased becomes even more critical.

LIMIT HEALING

The prevalence of instant Healing in a campaign
has a big effect on its overall lethality.
In "higher" fantasy instant combat
effective Healing is common, which allows
a group well stocked on healing magics and
/ or staffed with one or more healers to
survive and overcome dangers that would
annhilate less heal-heavy groups. There
is nothing intrinsically wrong with this
dynamic, and in fact is a deliberate staple
of some settings. However it is anathema
if you wish for more lethality.

By restricting or outright banning combat
effective Healing, you will instantly make
your games more lethal, by design. If you
restrict out-of-combat Healing as well your
game will become more long-term lethal as
well since characters will take much longer
to recover from encounters and will often
be carrying residual damage. However, take
care to ensure that you find the right balance;
many players do not enjoy having their character
get "gimped" and will lose interest
if they are constantly struggling with persistent
injuries to their PC's.